In multiple-track data-storage apparatus of the type in which the transducer head is stepped or otherwise moved on command to any of a series of parallel recording tracks on a record member, for example magnetic tape drives used in computer systems for data back-up and archival storage purposes, a variety of different positioning mechanisms are or may potentially be used. Most such systems employ stepper motors, which are very accurately controllable since they produce a particular and specific amount of output shaft rotation as a function of the number of excitation pulses applied, and such pulse actuation lends itself to very accurate control.
While a variety of different mechanisms are or may be used to apply the output shaft rotation of the stepper or other such motor to the transducer head itself, various forms of cams are typically employed and worm-drive or lead screw and follower systems are desirable due to the high degree of resolution and accuracy which such systems can provide, although such systems are sometimes difficult to implement due to physical layout and size constraints. Thus, the underlying conditions as well as the particular motion-transmitting structures and componentry used will be quite different in cases where the positioning motors may be located immediately adjacent the location of the transducer heads within the drive architecture, as opposed to cases where the positioning motor must be located at a substantial distance from the transducer head and its mounting structure.
In practically all instances where lead screw-type mechanisms are used for head-positioning in such data-storage applications, however, an ever-present problem which must be dealt with is that of indexing the follower travel along the lead screw, together with limiting follower travel at the end extremity thereof adjacent the drive motor (regardless of whether a stepper motor is employed, or some other form of servo motor). That is, the extent of lead screw follower travel along the lead screw thread can be accurately controlled if an index point is accurately established, but otherwise the follower will move until it ultimately encounters an obstacle, for example the motor housing or other such adjacent structure which is fixed in relation thereto, or simply runs off the end of the lead screw thread.
While it is true that such a condition of abutment will certainly limit the allowable travel of the follower along the lead screw, contact with a fixed stop such as the motor housing will inevitably produce a frictional binding of parts, including frictional jamming of the follower threads upon the lead screw threads and, usually, frictional wedging of the side of the follower against the motor housing or other such fixed structure as well. The result of either of these conditions is very undesirable, since it requires a high degree of motor torque to frictionally disengage (i.e., unjam) the wedged parts from one another, causing a number of significant problems in the positioning system. Furthermore, the specific point where follower travel is limited in this manner will vary from time-to-time, making the index point so generated inconsistent and unreliable, and thereby adversely affecting positioning accuracy during ensuing operation. That is, the drive must be able to generate a consistent and precise index point for the stepper drive electronics during initialization proceeding, but the desired accuracy and location of such an index point is not achieved when the lead screw follower is simply jammed against a stop at an end extremity. Further, of course, such jamming tends to overstress the lead screw and/or follower, and may well result in excess wear and/or early failure.